pBM^^H 

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401 

1//67 


Reprinted,  without  change  of  paging,  from  the  Journal  of  Heredity  (Organ  of  the  American  Genetic 
Association},  Vol.  VI,  No.  10,  Washington,  D.  C.,  U.S.A.,  October,  1915. 

UNIT  CHARACTERS 

Reality  of  Their  Existence  is  Fundamental  to  Study  of  Evolution,  But  Has  Never 
Been  Proved — Independent  Variability  of  Parts  and  Independent 
Transmissibility  of  Variations  Open  to  Question1 

S.  J.  HOLMES 
Associate  Professor  of  Zoology,  University  of  California,  Berkeley,  Cal. 


THE  doctrine  of  unit  characters 
is  one  that  has  figured  largely 
in  speculations  on  heredity  and 
evolution  from  the  time  of 
Darwin  to  the  present.  According  to 
this  doctrine  an  organism  is  a  sort  of 
mosaic  of  parts  each  of  which  is  depend- 
ent for  its  development  upon  some  kind 
of  discrete  entity  in  the  germ  cell. 

The  germ  cell  is  therefore  considered  a 
complex  of  organic  units  more  or  less 
independent  of  one  another  in  their 
activities  and  transmission. 

The  unit  character  hypothesis  is 
founded  on  (1)  the  assumed  independent 
variability  of  the  parts  of  an  organism, 
and  (2)  the  assumption  that  characters 
are  capable  of  independent  transmission. 
Independent  variability  was  appealed 
to  by  Darwin  in  support  of  his  hypo- 
thetical gemmules,  by  De  Vries  in  his 
Intracellular  Pangenesis,  and  especially 
by  Weismann  who  has  adduced  a 
formidable  array  of  facts  in  support 
of  this  doctrine  upon  which  he  founds 
much  of  his  argument  for  the  complex 
organization  of  the  germ  plasm. 

"There  are  human  families,"  says 
Weismann,  "in  which  individuals  occur 
repeatedly,  and  through  several  genera- 
tions, who  have  a  white  lock  of  hair,  in 
a  particular  spot,  on  an  otherwise  dark- 
haired  head.  This  cannot  be  referred 
to  external  influences,  it  must  depend  on 
a  difference  in  the  geim,  on  one,  too, 
which  does  not  affect  the  whole  body, 
not  even  all  the  hairs  of  the  body,  but 


only  those  of  a  particular  spot  on  the 
surface  of  the  head.  It  is  a  matter  of 
indifference  whether  the  white  coloring 
of  the  hair-tuft  is  produced  by  an 
abnormal  constitution  of  the  matrix  of 
the  hair,  or  by  other  histological  ele- 
ments of  the  skin,  as  of  the  blood- 
vessels or  nerves.  It  can  only  depend 
ultimately  on  a  divergently  constituted 
part  of  the  germplasm,  which  can  only 
affect  this  one  spot  on  the  head,  and 
alter  it,  if  it  is  itself  different  from  what 
is  usual.  On  this  account  I  call  -it  the 
determinant  of  the  relevant  skin-spot 
and  hair-group . ' ' 

"There  must  be  .as  many  of  these 
(determinants)  as  there  are  regions  in 
the  fully-formed  organism  capable  of 
independent  and  transmissible  varia- 
tion, including  all  the  stages  of  develop- 
ment." 

Weismann  has  no  quarrel  with  epige- 
nesis2  as  a  theoretic  possibility.  The 
complexity  of  the  germ  plasm  is  to  be 
measured  by  the  amount  of  independent 
variability  occuring  in  the  parts  of  the 
organism.  How  great  this  amount  is, 
how  many  parts  are  capable  of  under- 
going heritable  changes  independently 
of  the  others  is  a  question  to  be  answered 
only  through  extensive  observation, 
but  one  nevertheless  capable,  at  least 
theoretically,  of  being  answered. 

WEISMANN'S  ARGUMENT 

Weismann  argues  with  great  plausi- 
bility that  the  number  of  independently 


1  Read  before  a  joint  meeting  of  the  zoological  section,  American  Association  for  the  Advance- 
ment of  Science,  and  the  American  Genetic  Association,  at  Stanford  University,  August  4,  1915. 

2  Older  naturalists  imagined  that  a  minute  but  complete  embryo  must  be  preformed  and  in- 
cased in  either  the  egg  or  the  sperm.     In  1759  C.  F.  Wolff  enunciated  the  doctrine  of  epigenesis 
which,  modified  by  later  discoveries,  is  still  accepted  by   the  world  of  science.     As   at   present 
understood,  it  declares  that  there  is  no  pre-existence  of  an  organism  as  such,  but  that  the  embryo 
is  a  new  thing  created  as  the  result  of  the  union  of  egg  and  sperm  cells. — THE  EDITOR. 

473, 


3426G5 


474 


The  Journal  of  Heredity 


ssss 

c 


heritable  variations  presented  by  organ- 
isms must  be  great,  because  it  would  be 
impossible  to  have  complex  organs 
evolving  simultaneously,  as  they  ob- 
viously have  done,  unless  the  improve- 
ments in  the  one  did  not  modify  or 
interfere  with  improvements  in  the 
others.  If  every  variation  making  to- 
ward the  perfection  of  the  eye  were  tied 
lip  with  a  variation  in  the  ear,  the 
organs  of  digestion,  and  the  structure 
of  the  limbs,  it  seems  inevitable  that 
there  would  be  so  much  interference 
with  one  another's  progress  that  any 
progressive  evolution  of  a  number  of 
complex  organ  systems  would  be  prac- 
tically impossible.  Variations  accumu- 
lating toward  the  perfection  of  any  one 
organ,  argues  Weismann,  would  in  all 
probability,  work  toward  the  undoing 
of  various  other  organs.  Independent 
variability  of  parts  must,  therefore  be 
assumed  in  order  to  make  the  evolution 
of  a  complex  organization  possible 
through  variation  and  natural  selection. 

There  is  much  apparent  force  in  this 
argument  for  the  conception  of  the 
organism  as  a  mosaic  product.  Its 
real  weight  is  difficult  to  estimate, 
plausible  as  it  may  appear,  because  we 
know  so  little  of  the  possibilities  of 
organismal  variability.  However  the 
assumption  of  any  particular  kind  of 
variability  may  increase  or  lighten  the 
task  of  explaining  how  evolution  takes 
place,  it  is  obviously  our  first  duty  to 
inquire  whether  or  not  organisms  ac- 
tually vary  in  the  way  alleged.  Since 
so  much  has  been  built  upon  the  doctrine 
of  independent  variability  of  parts,  the 
the  burden  of  proof  may  fairly  be  held 
to  rest  with  those  who  espouse  this 
theory. 

Let  us  therefore  consider  some  of  the 
alleged  instances  of  independent  varia- 
tion. Take  the  classical  case  cited  by 
Weismann,  of  the  small  pit  in  the  ear 
which  ran  through  several  successive 
generations.  As  this  is  an  inherited 
character,  the  germ  plasm  of  the 
person  transmitting  it  must  be  slightly 
different  from  that  of  a' person  without 
this  defect.  But  does  it  follow  that 
"it  can  only  depend  ultimately  on  a 
•divergently  constituted  part  of  the 
.germplasm,  which  can  only  affect  this 


one  spot  on  the  head,  and  alter  it,  if  it 
is  itself  different  from  what  is  usual?" 
If  variations  such  as  this  could  come 
and  go,  leaving  the  rest  of  the  organism 
unmodified,  we  should  be  logically  led, 
I  believe,  to  adopt  Weismann 's  con- 
clusion that  these  variations  depend  on 
independent  carriers  of  some  sort  in  the 
germplasm.  Weismann's  reasoning  is 
good,  so  far  as  his  doctrine  of  deter- 
minants goes,  if  we  grant  his  funda- 
mental assumption.  If  a  small  pit  in 
the  ear  were  absolutely  the  sole  heredi- 
tary difference  between  two  human 
beings  we  might  be  forced  to  consider  it 
as  a  unit  character  depending  on  a 
special  determinant,  determiner,  or  other 
germinal  unit  or  entity.  But  do  we 
know  that  the  facts  are  as  Weismann 
assumes  ? 

STUDIES    NOT   DEEP    ENOUGH 

I  am  quite  sure  that  these  people 
with  a  pit  in  the  ear  have  never  been 
very  critically  studied  to  find  whether 
or  not  this  small  character  may  not  be  a 
mere  expression  of  more  general  dif- 
ferences in  constitution.  It  might  very 
well  be  that  this  pit  is  simply  a  relatively 
obvious  manifestation  of  a  very  slight 
difference  which  affects  the  organism  as 
a  whole.  The  same  may  be  true  of  the 
white  lock  of  hair  and  numerous  other 
characters  which  appear  to  vary  in- 
dependently of  the  rest  of  the  body. 

The  now  neglected  study  of  correlated 
variability  has  revealed  numerous  cases 
in  which  what  appear  as  single  varia- 
tions have  far-reaching  connections. 
Supernumerary  horns  in  sheep  are  said 
by  Youatt  to  go  along  with  great 
"length  and  coarseness  of  the  fleece." 
In  mammals  in  general  there  is  a  strong 
tendency  for  variation  to  affect  simul- 
taneously hair,  teeth  and  hoofs  or 
claws.  Darwin  points  out  that  the 
white  star  in  the  forehead  of  horses  is 
generally  correlated  with  white  feet, 
and  that  in  "  white^rabbits  and  cattle, 
dark  marks  often  co-exist  on  the  tips  of 
the  ears  and  on  the  feet."  |*oly- 
dactylism;  as  is  well  known,  tends  to 
affect  both  hands  and  feet.  How  are 
we  to  interpret  these  correlations? 
If  hands  and  feet  vary  together  do  the 
intervening  parts  of  the  skeleton  re- 


Holmes:  Unit  Characters 


475 


main  unaffected?  If  tip  of  ears  and 
hind  feet  show  parallel  variations  in 
color  does  it  not  suggest  that  we  are 
here  dealing  with  a  sort  of  outcropping 
of  a  color  variation  which  is  really 
present,  but  less  conspicuously  expressed 
in  other  parts  of  the  skin  ?  That  bodily 
changes  of  a  general  nature  may 
manifest  themselves  to  ordinary  ob- 
servation in  one  or  at  least  a  very  fewl 
characteristics  is  clearly  shown  in  the' 
effects  of  many  diseases.  Infectious 
diseases  may  have  their  characteristic 
symptoms  in  certain  form-changes  while 
leaving  the  rest  of  the  body  apparently 
unaffected.  Hutchinson's  teeth3  in 
children  for  instance  are  the  index  of  a 
general  bodily  disease  which  may  have 
no  other  very  obvious  sign. 

Introduce  some  toxin  of  disease  into 
the  body  and  you  produce  certain 
specific  characters.  Introduce  a  change 
affecting  all  the  cells  and  certain  parts 
only  will  reveal  the  fact  by  noticeable 
modifications.  The  appearance  of  in- 
dependent variability  of  parts  may  thus 
result  from  variations  that  are  in  reality 
organismal  in  their  extent.  Not  only 
have  so-called  particular  variations  not 
been  studied  sufficiently  to  establish  the 
fact  that  they  are  really  independent, 
but  numerous  cases  are  known  in  which 
variations  which  to  casual  observations 
would  seem  to  affect  but  a  single  part, 
are  nevertheless  correlated  with  minor 
changes  of  wide  extent.  We  contend 
therefore  that  the  alleged  independent 
variability  of  parts  upon  which  Darwin, 
De  Vries,  Weismann  and  others  have 
based  so  much  of  their  argument  for  the 
existence  of  discrete  germinal  units 
rests  upon  an  insecure  foundation. 

INDEPENDENT  TRANSMISSION 

The  question  of  the  independent 
transmission  of  characters  may  be  dealt 
with  more  briefly.  Owing  to  the  inde- 
pendent way  in  which  so-called  char- 
acters such  as  tallness  and  dwarf  ness, 
flower  color,  characters  of  seed  coat  and 
various  other  parts  of  peas  may  be 
separated  and  combined  almost  at  will 
according  to  the  fancy  of  the  breeder  it 


has  become  customary  to  look  upon 
these  characters  as  discrete  entities 
borne  by  discrete  elements  in  the  germ 
cells,  and  to  consider  the  organism  as  a 
mosaic  of  independently  heritable  parts. 
From  this  viewpoint  organisms  may  be 
likened  to  brick  buildings  in  which  the 
bricks  may  be  taken  out  and  replaced 
by  others  without  materially  affecting, 
except  secondarily,  the  bricks  which 
make  up  the  rest  of  the  structure.  But 
although  the  facts  of  Mendelian  in- 
heritance are  usually  interpreted  ac- 
cording to  the  mosaic  conception,  they 
do  not  I  believe  necessitate  the  adoption 
of  this  standpoint.  When  the  Anlage4 
of  a  green  pea  is  separated  from  that  of  a 
yellow  one  we  are  not  compelled  to 
assume  that  something  in  the  germ  cell 
that  stands  for  just  greenness  is  separ- 
ated from  something  that  is  the  repre- 
sentative of  mere  yellowness.  We  need 
assume  only  that  what  are  separated 
are  the  Anlagen  of  organisms  as  wholes 
possessing  the  characteristics  in  ques- 
tion. In  other  words  green  and  yellow 
represent  organismal  variations;  ex- 
pressed in  Weismannian  terms,  green 
and  yellow  depend  not  on  determinants, 
but  upon  ids,  the  hereditary  bases  of 
whole  organisms. 

The  logical  consequence  of  this  stand- 
point we  have  presented  is  that  all 
Mendelian  characters  are  really  general 
and  constitutional,  however  they  may 
appear  to  be  limited  to  a  particular 
feature  of  the  organism.  Many  Men- 
delian characters  are  quite  patently 
constitutional  while  others  are  ap- 
parently very  limited  in  their  extent 
like  pea  and  rose  comb  in  poultry. 
Attention  has  been  so  taken  up  .with 
characters  per  se  that  I  doubt  if  much 
careful  scrutiny  has  been  given  to  the 
possible  correlations  of  characters  in 
other  parts  of  the  body.  Has  anyone 
for  instance  very  carefully  looked  for 
any  more  general  attributes  which  may 
be  associated  with  pea  or  rose  comb,  or 
with  the  smooth  and  wrinkled  coats  of 
peas?  Correlations  may  be  difficult  to 
detect,  not  only  for  the  reasons  pre- 
viously mentioned,  but  because  the 


3  Hutchinson's  teeth  are  a  form  of  incisor  teeth  indicativa  of  jhereditary  syphilis. 

4  Anlage  is  a  German  term  much  used  by  genetists  to  denote  the  hypothetical  something  in 
the  germ-cells  which  determines  the  nature  of  a  given  part  of  the  adult  organism. — THE  EDITOR. 


476 


The  Journal  of  Heredity 


associated  characters  may  not  im- 
probably have  different  relations  of 
dominance  or  recessiveness  from  that  of 
their  more  obvious  correlates.  The 
question  which  we  have  raised  can,  at 
least  theoretically,  be  decided  by  obser- 
vation and .  experiment.  However  it  is 
decided  will  make  little  practical  dif- 
ference with  most  of  the  problems  that 
confront  the  investigator  in  genetics. 
But  there  are  certain  problems  of 
genetics,  I  suspect,  in  regard  to  which 
it  will  be  found  to  have  an  important 
bearing,  although  its  chief  importance 
is  in  the  way  it  influences  our  views  on 
certain  fundamental  problems  of  onto- 
geny and  evolution.  I  can  here  indicate 
but  a  few  cases  in  point : 

BEARING   ON   EVOLUTION 

Since  I  have  come  to  see  more  clearly 
the  implications  of  the  question  I  have 
discussed,  I  have  been  surprised  to  find 
how  many  of  the  difficulties  urged 
against  the  theory  of  natural  selection 
disappear  when  we  consider  variations 
as  organismal  instead  of  limited  pri- 
marily to  particular  parts.  Most  discus- 
sions, I  find,  consider  evolutionary 
problems  from  the  standpoint  of  the 
doctrine  of  unit  characters.  How  com- 
mon it  is  to  find  speculations  as  to  how 
this  or  the  other  character  could  have 
been  developed  through  natural  selec- 
tion, as  if  each  part  were  somehow 
separately  improved  by  a  series  of 
fortunate  survivals.  If  each  character 
is  considered  as  the  summation  of  a 
series  of  variations  which  primarily 
concern  that  character  alone,  and  if  the 
nature  of  the  variations  that  are 
integrated  is  determined  by  natural 
selection,  we  should  expect  most  attri- 
butes of  an  organism  to  be  of  a  useful 
kind.  If,  on  the  other  hand,  variations 


of  any  one  part  involve  variations1, 
throughout  the  organism,  then  the 
preservation  of  favorable  variations  in 
any  one  organ  would  of  necessity  entail 
changes  in  other  organs  which  for  the 
most  part  would  probably  have  no 
relation  to  utility.  On  this  view  a 
considerable  ingredient  of  non-adoptive 
characters  would  naturally  be  expected, 
and  it  is  probable  that,  through  correla- 
tion, parts  might  be  evolved  to  a  con- 
siderable degree  of  complexity  without 
having  any  important  use  in  the  life 
of  the  organism,  provided  they  did  not 
become  positively  dangerous  to  their 
possessors.  Much  of  the  evidence  ad- 
dt^qed  for  orthogenesis  is  what  we  should 
e:<  wet  to  find  if  evolution  occurred 
through  the  selection  of  organismal 
variations.  Much  of  the  difficulty  about 
the  beginnings  of  structures  and  their 
development  up  to  the  point  where  they 
acquire  selective  value  would,  I  believe, 
also  be  removed.  The  wonder  is  not  so 
much  that  selection  should  produce  a 
large  amount  of  what  Haeckel  would 
call  dysteleological  structures,  but  that 
it  is  able  to  produce  (if  we  grant  that  it 
does  produce)  so  much  that  is  so  nicely 
coadapted,  and  especially  that  it  is 
able  to  carry  on  the  simultaneous 
elaboration  and  perfection  of  numerous 
separate  systems  of  organs. 

In  these  days  of  attack  upon  evolu- 
tionary problems  through  direct  obser- 
vation and  experiment,  I  hope  I  may  be 
pardoned  for  presenting  anything  so 
atavistic  as  an  academic  discussion  of 
the  method  of  evolution.  But  even 
with  our  present  accumulation  of  facts 
bearing  on  this  much  discussed  problem 
there  is  still  something  to  be  gained  by 
reflection,  and  if  our  reflection  suggests 
new  things  to  look  for  it  will  assuredly 
not  be  in  vain. 


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